Wind tunnel investigations of the flutter characteristics of airfoils frequently require measurement of small oscillatory motions while the airfoil is under large steady-state loads. As the angle of attack is increased during wind tunnel tests, the steady-state load caused by the lift of the airfoil increases and must be balanced by opposing forces. The opposing forces are provided by the airfoil suspension system. The mass and stiffness properties of the suspension system are dictated by flutter scaling considerations. In general, the suspended, masses must be lightweight in order to simulate realistic mass/air density ratios and the suspension system stiffness low enough that flutter can occur within the wind tunnel operating envelope. Further, the model position in the wind tunnel test section must be maintained for large variations of steady aerodynamic loads. The two oscillatory modes of interest are the pitch mode, involving the twisting about an axis along the span, and the plunge mode involving the up and down flapping motion of the airfoil.
A variety of methods of suspension have been used in an effort to meet these conflicting requirements for low stiffness and high load carrying capability. In contrast, testing for steady-state characteristics, such as lift or drag coefficients, has typically required stiff suspension system to carry the higher loads generated by high lift forces.
It is common practice in wind tunnel flutter tests to set the model at low angles of attack so as to reduce steady state aerodynamic loads and allow soft suspension systems. Unfortunately, flutter is often more critical at higher angles of attack where large steady state forces are also present. Because of this need for flutter model testing at high lift conditions, a novel suspension arrangement was previously developed by the National Aeronautics and Space Administration using a pneumatic cylinder to provide the large steady-state download to react against lift forces on the wind tunnel model. In order to achieve this lift-compensation force without affecting the dynamics of the suspension system, a complex blow-by system was designed into the pneumatic cylinder and two large accumulator-reservoirs were used. Although this suspension does provide for large steady-state loads while retaining proper dynamic characteristics, the complexity and size of the system, along with the considerable volume of compressed gas blow-by necessary to operate the system, results in excessive expense and maintenance and requires lengthy set-up times. In order to avoid these difficulties, a development project was initiated to determine an alternate means of achieving the desired suspension system characteristics. The present invention is a product of that project and is intended to overcome prior art shortcomings. It is, therefore, an object of the present invention to provide a wind tunnel suspension system which can support an airfoil and is adjustable in angle of attack.
Yet another object of the present invention is to incorporate a self-alignment mechanism into the suspension system, permitting the airfoil to remain in a fixed position despite changes in airload.
A further object of the present invention is to have the system provide a soft spring restraint under all steady-state load conditions.
Still another object of the present invention is to provide a compound spring suspension system which will simultaneously provide low plunge stiffness while requiring relatively small static plunge-spring deflections to counteract steady state lift forces and which will permit variable pitch and plunge frequencies, changeable airfoil rotation axes, and a self-aligning system, to maintain a constant mean position of the test model with changing airloads.